Semiconductor memory device

ABSTRACT

A memory device includes an error detection and correction system with an error correcting code over GF(2 n ) wherein the system has an operation circuit configured to execute addition/subtraction with modulo 2 n −1, and wherein the operation circuit has a first operation part for performing addition/subtraction with modulo M and a second operation part for performing addition/subtraction with modulo N (where, M and N are integers which are prime with each other as being obtained by factorizing 2 n −1), and wherein the first and second operation parts perform addition/subtraction in parallel to output an operation result of the addition/subtraction with modulo 2 n −1.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims the benefit of priority from theprior Japanese Patent Application No. 2006-042250, filed on Feb. 20,2006, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor memory device and, moreparticularly, to an on-chip error detection and correction systemadaptable for use therein.

2. Description of the Related Art

Electrically rewritable nonvolatile semiconductor memory devices, i.e.,flash memories, increase in error rate with an increase in number ofdata rewrite operations. In particular, the quest for larger storagecapacity and further enhanced miniaturization results in an increase inerror rate. In view of this, an attempt is made to mount or “embed” abuilt-in error correcting code (ECC) circuit on flash memory chips or,alternatively, in memory controllers for control of these chips. Anexemplary device using this technique is disclosed, for example, inJP-A-2000-173289.

A host device side using more than one flash memory is designable tohave an ECC system which detects and corrects errors occurring in theflash memory. In this case, however, the host device increases in itsworkload when the error rate increases. For example, it is known that atwo-bit error correctable ECC system becomes greater in calculationscale, as suggested by JP-A-2004-152300.

Accordingly, in order to cope with such error rate increase whilesuppressing the load increase of the host device, it is desired to builda 2-bit error correctable ECC system in the flash memory. What is neededin this case is to meet the conflicting requirements: i.e., increasingthe ECC system's arithmetic operation speed, and yet lessening possiblepenalties of read/write speed reduction of the flash memory.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided asemiconductor memory device including an error detection and correctionsystem with an error correcting code over Galois field GF(2^(n)),wherein

the error detection and correction system includes an operation circuitconfigured to execute addition/subtraction with modulo 2^(n)−1, andwherein

the operation circuit includes a first operation part for performingaddition/subtraction with modulo M and a second operation part forperforming addition/subtraction with modulo N (where, M and N areintegers, which are prime with each other as being obtained byfactorizing 2^(n)−1), which perform addition/subtraction simultaneouslyin parallel with each other to output an operation result of theaddition/subtraction with modulo 2^(n)−1.

According to another aspect of the present invention, there is provideda semiconductor memory device including a cell array with electricallyrewritable and non-volatile memory cells arranged therein, and an errordetection and correction system, which is correctable up to 2-bit errorsfor read out data of the cell array by use of a BCH code over Galoisfield GF(256), wherein

the error detection and correction system includes:

an encoding part configured to generate check bits to be written intothe cell array together with to-be-written data;

a syndrome operating part configured to execute syndrome operation forread out data of the cell array;

an error location searching part configured to search error locations inthe read out data based on the operation result of the syndromeoperating part; and

an error correcting part configured to invert an error bit in the readout data detected in the error location searching part, and output it,and wherein

the error location searching part includes an operation circuitconfigured to execute index addition/subtraction with modulo 255, andwherein

the operation circuit includes a first operation part for performingaddition/subtraction with modulo 17 and a second operation part forperforming addition/subtraction with modulo 15, which performaddition/subtraction simultaneously in parallel with each other tooutput an operation result of the index addition/subtraction with modulo255.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of main part of a flashmemory in accordance with an embodiment of this invention.

FIG. 2 is a diagram showing a detailed arrangement of a cell array inthe flash memory.

FIG. 3 is a diagram showing a further detailed configuration of the cellarray.

FIG. 4 is a diagram showing a data level relationship of the flashmemory.

FIG. 5A is a diagram showing a 4-bit parity check circuit as used in anECC circuit for performing even/odd judgment of “1”; and FIG. 5B is adiagram showing symbols of the parity checker circuit.

FIG. 6 is a diagram showing a product computation method of polynomialsover GF(2).

FIG. 7 is a diagram showing a parity check circuit for use in suchproduct computation.

FIG. 8 is a diagram showing a check bit calculation method in theencoding part in the ECC circuit.

FIG. 9 is a diagram showing a calculation method of a syndromepolynomial S₁(x) in the decoding part in the ECC circuit.

FIG. 10 is a diagram showing a calculation method of syndrome polynomialS₃(x³) in the same.

FIG. 11 is a diagram showing 144 degrees as selected from an informationpolynomial in order to be used as data bits.

FIG. 12 is a table of “n”s with the coefficient of each degree of the15-degree remainder polynomial being of “1”.

FIG. 13 is a diagram showing a configuration of a parity check circuitfor use in check bit calculation.

FIG. 14 is a table of n's with the coefficient of each order of “1” atthe selected n's of remainder polynomial p_(n)(x) as used in calculationof the syndrome polynomial S₁(x).

FIG. 15 shows an exemplary circuit for calculation of the syndromepolynomial S₁(x).

FIG. 16 is a table of n's with each degree coefficient “1” at chosen n'sof a remainder polynomial p^(3n)(x) for use in calculation of a syndromepolynomial S₃(x³).

FIG. 17 shows an exemplary circuit for calculation of the syndromepolynomial S₃(x³).

FIG. 18 is a coefficient table of remainder polynomial p^(n)(x) as needin decoding.

FIGS. 19A and 19B are tables for showing the relationships betweenindexes n and y_(n).

FIG. 20 shows the summarized relationships between n and y_(n).

FIG. 21 is a table showing classification of index y_(n) based on15y_(n)(17) and 17y_(n)(15).

FIG. 22 is a table showing the relationships between index i and thecorresponding physical data bit position k.

FIG. 23A shows an index rotator for performing addition/subtraction ofindexes; and FIG. 23B circuit symbol thereof.

FIG. 24 shows the index rotator part for operating the first congruencein Expression 18.

FIG. 25 shows the decode circuit used in FIG. 25.

FIG. 26 is a table showing index n as the remainder class index 15n(17).

FIG. 27 is a table showing index n as the remainder class index−45n(17).

FIG. 28 shows the index rotator part for operating the second congruencein Expression 18.

FIG. 29 is a table showing index n as the remainder class index 17n(15).

FIG. 30 is a table showing index n as the remainder class index−17n(15).

FIG. 31 shows output buses for integrating outputs of the index rotatorsshown in FIGS. 24 and 28.

FIG. 32 shows the index rotator part for operating the first congruencein Expression 19.

FIG. 33 shows the decode circuit used in FIG. 32.

FIG. 34 is a table showing the relationships between 15y_(n)(17),17y_(n)(15) and 15n(17).

FIG. 35 shows the index rotator part for operating the second congruencein Expression 19.

FIG. 36 is a code table of the decode circuit in FIG. 35.

FIG. 37 shows error correction part for integrating the outputs of indexrotators shown in FIGS. 32 and 35 to output error bit location signals.

FIG. 38 is a table showing the relationships between 15i(17), 17i(15), iand k.

FIG. 39 shows the error correction circuit.

FIG. 40 shows the 2-bit parity check circuit used in FIG. 39.

FIG. 41 is a diagram showing a configuration of the ECC circuit relatingto one block memory core.

FIG. 42 shows another memory core configuration.

FIG. 43 shows another embodiment applied to a digital still camera.

FIG. 44 shows an internal configuration of the digital still camera.

FIGS. 45A to 45J show other electric devices to which the embodiment isapplied.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Some embodiments of this invention will be described with reference tothe accompanying figures of the drawing below.

Referring to FIG. 1, there is illustrated a block diagram showing mainparts of a flash memory embodying the invention. This flash memoryincludes a couple of banks BNK0 and BNK1. Bank BNK0 is formed of fourseparate memory cell arrays 1—i.e., T-cell array (0-1), C-cell array(0-2), T-cell array (0-3), and C-cell array (0-4). The other bank BNK1also is formed of four cell arrays 1, i.e., T-cell array (1-1), C-cellarray (1-2), T-cell array (1-3), and C-cell array (1-4).

Each cell array 1 is associated with a row decoder (RDEC) 3 forperforming word-line selection. Sense amplifier (SA) circuits 2 areprovided, each of which is commonly owned or “shared” by neighboringT-cell array and C-cell array. These T-cell and C-cell arrays have aplurality of information cells, T-cells and C-cells, respectively, eachhaving at least one reference cell R-cell as will be described later indetail.

The information cells T-cell and C-cell are the same in structure as thereference cell R-cell. Upon selection of an information cell T-cell fromT-cell array, a reference cell R-cell is selected from a C-cell arraythat makes a pair with this T-cell array. Similarly, when an informationcell C-cell is selected from C-cell array, a reference cell R-cell isselected from a T-cell array that makes a pair therewith.

Read/write data of upper and lower cell array groups each having fourseparate cell arrays 1 are transferred between sense amplifier circuits2 and external input/output (I/O) nodes through data buses 5 and 6 viaan I/O buffer 7. Between the upper and lower cell array groups eachhaving four cell arrays 1, an error correcting code (ECC) circuit 8 isprovided as an error detection and correction system, which is operableto detect and correct errors of read data.

Referring next to FIG. 2, a detailed configuration of a pair of T-cellarray and C-cell array with their shared sense amp circuit 2 is shown.T-cell array has parallel extending bit lines BL, each of which isconnected to a plurality of information cell NAND strings, T-NAND, andat least one reference cell NAND string, R-NAND. Similarly C-cell arrayhas parallel bit lines BBL, to each of which are connected a pluralityof information cell NAND strings, C-NAND, and at least one referencecell NAND string, R-NAND. Bit line BL of T-cell array and itscorresponding bit line BBL in C-cell array constitute a pair.

The sense amp circuit 2 has sense units SAU of a current detection type,each of which is for detecting a difference between currents flowing ina pair of bitlines BL and BBL to sense data. Although in FIG. 2 a senseunit SAU is disposed one by one between paired bit lines BL and BBL,there is usually employed a scheme for causing a single sense unit to beshared by more than two bit line pairs.

A prespecified number of serial combinations of the information cellNAND strings T-NAND, C-NAND and reference cell NAND string R-NAND arelaid out in the direction at right angles to the bitlines, therebyconstituting cell blocks. In the respective cell blocks, word lines TWL,CWL and RWL are disposed. More specifically, these cell blocks each hasa plurality of groups of parallel word lines which insulatively cross or“intersect” the bit lines BL and BBL—i.e., bundles of word lines TWLassociated with respective columns of information cell NAND stringsT-NAND, sets of wordlines CWL connected to respective columns ofinformation cell NAND strings C-NAND, and wordlines coupled to eachcolumn of reference cell NAND strings R-NAND.

FIG. 3 shows a detailed configuration of circuitry which includes asense unit SAU and an information cell NAND string T-NAND (or C-NAND)and a reference cell NAND string R-NAND as connected to the sense unit.Each of these NAND strings has a serial connection of electricallyrewritable nonvolatile memory cells M0 to M31 and a couple of selecttransistors SG1 and SG2 at opposite ends thereof. While the nonvolatilememory cells M0-M31 used are the same in transistor structure, these mayfunction as the information cells T-cell (or C-cell) in information cellNAND string T-NAND (or C-NAND) or act as the reference cell R-cell inreference cell NAND string R-NAND.

In a data sense event, a memory cell of the information cell NAND stringT-NAND (or C-NAND) and its corresponding cell in reference cell NANDstring R-NAND are selected together at a time. This simultaneousdata/reference cell selection results in currents Ic and Ir flowing inthese cell strings, respectively. The sense unit SAU detects adifference between these cell currents Ic and Ir to sense data.

FIG. 4 shows a distribution of memory cell data levels (thresholdlevels) in case of four-level storage scheme. Any one of four differentdata levels L0, L1, L2 and L3 is written into an information cellT-cell, C-cell. A reference level Lr is written into reference cellR-cell, which level is set to have a potential between the data levelsL0 and L1 for example.

Bit allocation of four data levels L0-L3 is different between theinformation cells T-cell and C-cell. For example, suppose thatfour-level data is represented by (HB, LB), where HB is the upper-levelbit; and LB lower-level bit. In an information cell T-cell on the T-cellarray side, data bits are assigned as follows: L0=(1,0), L1=(1,1),L2=(0,1), and L3=(0,0). While, in an information cell C-cell on theC-cell array side, data bits are assigned as follows: L0=(0,0),L1=(0,1), L2=(1,1), and L3=(1,0).

In FIG. 4, there are shown read voltages R1 to R3 given to informationcell T-cell, C-cell during reading in a way pursuant to the data to beread, and a read voltage Rr applied to reference cell R-cell. Also shownhere are write-verify voltages P1 to P3 to be given to the informationcell T-cell, C-cell during write-verifying, and a write-verify voltagePr applied to the reference cell R-cell.

Although, in the example, four-level data storage scheme has beenexplained, it will be used in general such a multi-level data storagescheme that two or more bits are stored in each memory cell.

An explanation will now be given of a technique used in this embodimentfor mounting the ECC circuit 8 and its access mode in a case where a 1gigabit (Gb) of memory is configured using two 512-megabit (Mb) banksBNK0 and BNK1.

The memory as discussed here is of a x16IO configuration, in whichaddress generation is in common to all the banks although each bank'spage address is settable independently and wherein allocation is made toeach bank by designating which page address is applied to which bank.Accordingly, interleaving is done between the banks for page addressutilization.

Each bank has 1,024 k pages. The page length defined in common for 16IOs of each bank is 32 bits, which may be output 8 bits by 8 bits in aparallel way. The page length is the maximum or “longest” data lengthper bank with respect to one IO readable by a sense operation withone-time wordline address setting.

With such an arrangement, 128 bits of data are output outside byone-time data transmission from the sense amp circuit 2. In other words,this design permits 2^(k) data bits to be transferred together, where kis an integer.

The ECC circuit 8 built in the flash memory is arranged to employBose-Chaudhuri-Hocquenghem (BCH) code with 2-bit errorcorrectability—that is, double error correction BCH code system, whichwill be referred to as “2EC-BCH” code hereinafter. To enable 2-bit errorcorrection, a need is felt to use a simultaneous equation havingdifferent roots. The 2EC-BCH code is a cyclic code generated with a codegeneration polynomial as denoted by a product of two primitivepolynomials.

The bit length used here is given by 2^(n)−1. Data bits usable asinformation are 2^(n)−1−2n bits. To deal with 2^(m) bits of data, letn=m+1. This makes it inevitable to use a data length that isapproximately two times greater than the quantity required.

In the memory configuration of FIG. 1, 2EC-BCH code used for executionof 2-bit error correction in 128-bit information data is one over Galoisfield GF(2⁸). In this case, usable bit length is 2⁸−1=255, whichrequires the use of 16 bits as error check bits. Thus, in case 144 bitsare used for 16 check bits and 128 information data bits, the remaining111 bits become extra ones. In this case, as shown in FIG. 1, the databuses 5 extending from upper and lower cells which bisect each bank are72 DQ lines, respectively, resulting in a total of 144 bits of databeing transferred together at a time.

The ECC system is variable in efficiency depending upon the handling ofuseless extra 111 bits and how information bit selection is performed inthe BCH code system. Thus, it is necessary to take into consideration amethod for configuring the best suited ECC system.

Although in some cases the required bit number becomes greater than 128bits when considered also including the redundancy for replacement of adefective cell(s), this may readily be analogously extendable from thecase of the 128-bit data length discussed here. In general, the number Lof data bits to be error-corrected in the 2EC-BCH system is selected inthe range of L≦255−16=239.

(Data Encoding)

An explanation will be given of the outline of 2EC-BCH over Galois fieldGF(2⁸). Letting a primitive root (element) of GF(256) be α, 8-degreeprimitive polynomial m₁(x) on the ground field GF(2) with this element abeing as its own root is represented by Expression 1. In other words,irreducible polynomials of a power of α and a power of x due to m₁(x)become mutually corresponding elements in GF(256).

Additionally, as an 8-degree irreducible polynomial with a cubic of abeing as its root, polynomial m₃(x) that is relatively prime with m₁(x)is used as shown in the following Expression 1.

α:m ₁(x)=x ⁸ +x ⁴ +x ³ +x ²+1

α³ :m ₃(x)=x ⁸ +x ⁶ +x ⁵ +x ⁴ +x ² +x+1  [Exp. 1]

Based on these two primitive polynomials, a 2-bit error correctable ECCsystem (i.e., 2EC-BCH code system) will be configured. To performencoding with check bits added to the data being written, prepare aproduct polynomial g(x) of m₁(x) and m₃(x) as a code generationpolynomial, as shown in Expression 2 below.

$\begin{matrix}\begin{matrix}{{g(x)} = {{m_{1}(x)}{m_{3}(x)}}} \\{= {x^{16} + x^{14} + x^{13} + x^{11} + x^{10} + x^{9} + x^{8} + x^{6} + x^{5} + x + 1}}\end{matrix} & \left\lbrack {{Exp}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

A maximal number of two-bit error correctable bits capable of beingutilized as information bits is 239, which is obtained by subtractingcheck bit numbers, 16, from 2⁸−1(=255). Based on these bits, whileletting coefficients from bit positions 16 to 254 be a₁₆ to a₂₅₄, make a238-degree information polynomial f(x) as indicated by Expression 3.

f(x)=a ₂₅₄ x ²³⁸ +a ₂₅₃ x ²³⁷ + . . . +a ₁₈ x ² +a ₁₇ x+a ₁₆  [Exp. 3]

For example, 128 bits are actually used in the 239 terms. In this case,letting the remaining coefficients of 111 bits be fixed to “0”, theinformation polynomial becomes one with the lack of those terms ofcorresponding degrees. Depending upon which degree numbers are selectedas the 111 terms with such “0”-fixed coefficients from the informationpolynomial f(x) having 239 terms, the computation amount of syndromecalculation becomes different, which is to be executed during decodingas described later. Thus, this selection technique becomes important.This will be explained later.

From the information polynomial f(x), form a data polynomial f(x)x¹⁶that contains 16 check bits. To make such check bits from this datapolynomial, the data polynomial f(x)x¹⁶ will be divided by the codegeneration polynomial g(x) to obtain 15-degree remainder polynomial r(x)as shown in the following Expression 4.

f(x)x ¹⁶ =q(x)g(x)+r(x)

r(x)=b ₁₅ x ¹⁵ +b ₁₄ x ¹⁴ + . . . +b ₁ x+b ₀  [Exp. 4]

Use the coefficients b₁₅ to b₀ of this remainder polynomial r(x) as thecheck bits. In other words, 128 coefficients a_(i(128)) to a_(i(1))selected from 239 ones serve as “information bits” while 16 bits of b₁₅to b₀ serve as “check bits”, thereby resulting in that a total of 144bits become “data bits” to be stored in the memory as shown in thefollowing Expression 5.

a_(i(128))a_(i(127)) . . . a_(i(3))a_(i(2))a_(i(1))b₁₅b₁₄ . . .b₁b₀  [Exp. 5]

Here, a_(i(k)) is the data to be externally written into the memory.Based on this data, a check bit b_(j) is created in the chip-embeddedECC system, which bit will be simultaneously written into the cellarray.

(Data Decoding)

Next, an explanation will be given of a method for detecting errors from144 bits of data read out of the cell array and for correcting up to 2bits of errors.

Supposing that errors take place when the memory stores the coefficientsof 254-degree data polynomial f(x)x¹⁶, such errors also are expressed by254-degree polynomial. This error polynomial being e(x), the data readfrom the memory may be given by a polynomial ν(x) with a structure shownin Expression 6 as follows.

ν(x)=f(x)x ¹⁶ +r(x)+e(x)  [Exp. 6]

A term with the coefficient of this error polynomial e(x) in Expression6 being at “1” is identical to an error. In other words, detecting e(x)is equivalent to performing error detection and correction.

What is to be done first is to divide the readout data polynomial ν(x)by the primitive polynomials m₁(x) and m₃(x) to obtain the respectiveremainders, which are given as S₁(x), S₃(x). As shown in Expression 7,it is apparent from the structure of ν(x) that each is equal to theremainder of e(x) divided by m₁(x), m₃(x).

$\begin{matrix}{{{{v(x)} \equiv {{S_{1}(x)}\mspace{14mu} {mod}\mspace{14mu} {m_{1}(x)}}}\mspace{20mu}->{{e(x)} \equiv {{S_{1}(x)}\mspace{14mu} {mod}\mspace{14mu} {m_{1}(x)}}}}{{{v(x)} \equiv {{S_{3}(x)}\mspace{14mu} {mod}\mspace{14mu} {m_{3}(x)}}}\mspace{20mu}->{{e(x)} \equiv {{S_{3}(x)}\mspace{14mu} {mod}\mspace{14mu} {m_{3}(x)}}}}} & \left\lbrack {{Exp}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

These division remainders S₁(x) and S₃(x) are referred to as syndromepolynomials.

Assuming that 2-bit errors are present at i-th and j-th bits, e(x) willbe expressed as follows: e(x)=x^(i)+x^(j). These values i and j areobtainable by calculation of the index “n” of x=α^(n), i.e., a root ofm₁(x) that is an element in GF(256). More specifically, when letting aremainder, which is obtained by dividing x^(n) by m₁(x), be p^(n)(x),α^(n)=p^(n)(x). As shown in the following Expression 8, let α^(i) andα^(i) corresponding to error degrees be X₁ and X₂, respectively; let theindexes corresponding to S₁(α) and S₃(α³) with respect to syndromesS₁(x) and S₃(x) be σ₁ and σ₃; and let S₁(α) and S₃(α³) be S₁ and S₃,respectively.

X ₁ =p ^(i)(α)=α^(i)

X ₂ =p ^(j)(α)=α^(j)

S ₁(α)=S ₁=α^(σ1)

S ₃(α³)=S ₃=α^(σ3)  [Exp. 8]

Since m₃(α³)=0, we obtain the following Expression 9.

S ₁ =X ₁ +X ₂ =e(α)

S ₃ =X ₁ ³ +X ₂ ³ =e(α³)  [Exp. 9]

At the second stage, consider polynomial Λ^(R)(x) with unknownquantities X₁ and X₂ as its roots, product X₁X₂ is representable by S₁and S₃ as in Expression 10, so that the coefficients involved arecalculable from the syndrome polynomials.

$\begin{matrix}{\begin{matrix}{{S_{3}/S_{1}} = {\left( {X_{1}^{3} + X_{2}^{3}} \right)/\left( {X_{1} + X_{2}} \right)}} \\{= {X_{1}^{2} + {X_{1}X_{2}} + X_{2}^{2}}} \\{= {\left( {X_{1} + X_{2}} \right)^{2} + {X_{1}X_{2}}}} \\{= {S_{1}^{2} + {X_{1}X_{2}}}}\end{matrix}{{X_{1}X_{2}} = {\left( {S_{3} + S_{1}^{3}} \right)/S_{1}}}\begin{matrix}{{\Lambda^{R}(x)} = {\left( {x - X_{1}} \right)\left( {x - X_{2}} \right)}} \\{= {x^{2} + {S_{1}x} + {\left( {S_{3} + S_{1}^{3}} \right)/S_{1}}}} \\{= {x^{2} + {\alpha^{\sigma 1}x} + \alpha^{{\sigma 3} - {\sigma 1}} + \alpha^{2{\sigma 1}}}}\end{matrix}} & \left\lbrack {{Exp}.\mspace{14mu} 10} \right\rbrack\end{matrix}$

At the third stage, finding α^(n), i.e., a root of Λ^(R)(x) in GF(256),it becomes possible to obtain the error bit locations i and j as “n” ofα^(n) from X₁, X₂=α^(n). In other words, searching Λ^(R)(α^(n))=0 forn=0, 1, 2, . . . , 254, a hit number “n” will be specified as an errorbit.

As shown in Expression 11 below, in case of a 1-bit error, we obtainX₁=S₁, X₁ ³=S₃=S₁ ³. Thus, the error location is defined from S₁. Ifthere are no errors, we obtain S₁=S₃=0. In case there are 3 bits or moreerrors and its position is incomputable, either one of S₁ and S₃ becomes0.

(a) If 1-bit error, X₁=S₁ and X₁ ³=S₃=S₁ ³.

(b) If 0-bit error, S₁=S₃=0.

(c) If more than 3-bit error, S₁ or S₃ is equal to 0.  [Exp. 11]

As described above, error location searching is for obtaining index “n”of α^(n) that satisfies Λ^(R)(x)=0. For the purpose, in this embodiment,change Λ^(R)(x) shown in Expression 10, and make possible to obtain “n”by use of only index relationships. In detail, using the conversion of:x=α^(σ1)y, to solve Λ^(R)(x)=0, and to obtain variable y shown in thefollowing Expression 12, it becomes equal to each other.

y ² +y+1+α^(σ3−3σ1)=0  [Exp. 12]

By use of this Expression 12, directly comparing the index obtained byvariable calculation with that defined by syndrome calculation, it ispossible to find a coincident variable. In detail, to solve theExpression 12, substitute α^(n) for y to obtain the index y_(n) shown inExpression 13.

y ² +y+1=α^(2n)+α+1=α^(yn)  [Exp. 13]

As shown in the following Expression 14, comparing the index σ₃−3σ₁obtained by the syndrome calculation with that y_(n) obtained by thevariable calculation, coincident “n” becomes the index of ycorresponding to the error location.

σ₃−3σ₁ ≡y _(n) mod 255  [Exp. 14]

To restore the index of variable y to that of the real variable x, asshown in Expression 15, multiply α^(σ1) into y.

x=α ^(σ1) y=α ^(σ1+n)  [Exp. 15]

The index σ₁+n of a as shown in Expression 15 is that of x correspondingto the error location, and this x will satisfy the equation of:Λ^(R)(x)=0.

Essentials for execution of actual calculations are summarized asfollows.

What is needed in encoding is a remainder table, i.e., a table ofcoefficients of remainder polynomial r(x), which is generated by codegeneration polynomial g(x) from 128 terms selected as data bits from thedata polynomial f(x)x¹⁶ of degree 254 in maximum. For check bitcalculation, select those coefficients corresponding to the databit-selected terms, followed by execution of addition over GF(2) usingtwo-element code of “0” or “1”.

In decoding, when performing calculation of the syndrome polynomialsS₁(x) and S₃(x³), a remainder table is necessary, which is a table ofcoefficients of remainder p^(n)(x) obtained by primitive polynomialm₁(x) from 254 to 0 degree. Based on this table, the calculation is doneas similar to the check bit calculation.

To shorten an ECC calculation time in the memory system which does notuse all of the 239 data bits usable in 2EC-BCH using GF(256), it is inneed of employing a practical selection method for selecting actuallyused terms (degrees) from the information polynomial. Especially for thesyndrome calculation, it is necessary to choose specific terms (degrees)capable of efficiently obtaining the remainder.

An explanation will first be given of a practical remainder calculationmethod.

In calculations over GF(2), both multiplication and division are carriedout by addition of polynomial coefficients—that is, based on even/oddjudgment of the numbers of “1”. Thus a calculator circuit used here isprincipally designed to perform parity check in a way as follows: if thenumber of terms, coefficient of which is “1”, is an even number, outputa computation result “0”; if it is an odd number then its calculationresult becomes “1”.

A 4-bit parity check (PC) circuit with a simplified configuration isshown in FIG. 5A, a circuit symbol of which is shown in FIG. 5B. Whileparity checkable circuitry with various configurations is designable andmodifiable on a case-by-case basis, the illustrative circuit is arrangedto have parity-check logic units made up of transistors only forpurposes of convenience in illustration and discussion herein.

This parity check circuit includes an upper-stage circuit 401 whichreceives at its inputs four bits a, b, c, d and their complementarysignals /a, /b, /c, /d and generates “0” at an output node EPexclusively when an even number of “1”s are in the input signals, and alower-stage circuit 402 which generates “0” at its output node OP onlywhen an odd number of “1”s are in the input signals thereof. Thesecircuits 401-402 are formed of eight parallel-connected gates betweenthe power supply voltage Vdd and ground potential Vss, each of whichgates has four 4-bit input PMOS transistors and NMOS transistors.

More specifically, the upper-stage circuit 401 is such that when its “1”inputs are 0, 2 or 4 in number, NMOS transistors on the Vss side arerendered conductive, causing the EP's potential to be “0” (EP=“0”). Forthe lower-stage circuit 402, when the number of its inputs “1”s is 1 or3, the Vss-side NMOS transistors turn on, resulting in establishment ofOP=“0”.

With this parity check circuit, 4-bit parity check is calculable withina shortened delay time, which is equivalent to that of a single stage ofinverter.

A calculation method of a product over GF(2) of the polynomial inGF(256) using the above-described parity check circuit is shown in FIG.6. Every arithmetic processing of the polynomial in GF(256) becomescomputation between 7-degree polynomials p^(n)(x) because the primitivepolynomial is of degree 8. Thus, every calculation result of four basicoperations—i.e., addition, subtraction, multiplication anddivision—becomes any one of the polynomials p^(n)(x). Division{p^(n)(x)}⁻¹ becomes a product of p^(255−n)(x).

While letting the coefficient of m-degree term of p^(n)(x) berepresented by P^(n) _(m), a product of 7-degree polynomials p^(i)(x)and p^(j)(x) in Expression 16 below is calculated as shown in FIG. 6.Obviously, the coefficient P^(n) _(m) is either “0” or “1”.

p ^(i)(x)=P ^(i) ₇ x ⁷ +P ^(i) ₆ x ⁶ +P ^(i) ₅ x ⁵ +P ^(i) ₄ x ⁴ +P ^(i)₃ x ³ +P ^(i) ₂ x ² +P ^(i) ₁ x+P ^(i) ₀

p ^(j)(x)=P ^(j) ₇ x ⁷ +P ^(j) ₆ x ⁶ +P ^(j) ₅ x ⁵ +P ^(j) ₄ x ⁴ +P ^(j)₃ x ³ +P ^(j) ₂ x ² +P ^(j) ₁ x+P ^(j) ₀  [Exp. 16]

-   -   -   As x^(n)≡p^(n)(x) (mod m₁(x)), use such a rule that the            14-degree term is with p^(j+7) appearing 7-item ahead of the            multiplying p^(j)(x). Then, the product of polynomials            p^(i)(x) and p^(j)(x) is enabled by multiplication and            addition (i.e., parity check) between the respective            polynomial coefficients as shown in FIG. 6.

In other words, parity check is done which is per-degree addition offrom 7 to 0 of 1- or 0-multiplied ones, and its result becomes thecoefficient of each degree of p^(i)(x)p^(j)(x). Regarding themultiplying p^(j)(x), there are required those polynomial coefficientsup to a 7-degree ahead one.

The circuitry required here may be the parity check circuit only, withadditional provision of a circuit for performing inputting unique to thecoefficients of from the multiplying p^(j)(x) to p^(j+7)(x). This isbecause no parity check is needed when the coefficient P^(j) _(m)=0. Asshown in FIG. 7, the parity check circuit is modifiable to employ 3-bitparity check circuit, 2-bit parity check circuit other than the 4-bitparity check circuit in accordance with input number needed.

This computation method is applied upon generation of the check bitsduring encoding, which will be explained below.

The check bit calculation is performed as follows: dividing datapolynomial f(x)x¹⁶ generated from information data by code generationpolynomial g(x), thereby obtaining remainder polynomial r(x). To performthis arithmetic operation, precalculate coefficients of 15-degreeremainder polynomial r^(i)(x) which was obtained by dividing a singleterm x^(i) by g(x), and use them in a similar way to the case of themultiplication of p^(n)(x). The coefficient of r^(i)(x) is referred toas R^(i) _(m)(m=0, 1, 2, . . . , 15), and the coefficient of x^(i) off(x)x¹⁶, which serves as an information bit, is referred to as a_(i).

Actual calculation is as follows. Assuming that data bits are a₁₆ toa₂₅₄, 111 bits out of them are selected in a way such that the syndromecalculation to be done later becomes less, and then fixed to “0”. Next,as shown in FIG. 8, perform multiplication of the data polynomial'scoefficient a_(i) and the remainder polynomial's coefficient R^(i) _(m)and addition (parity check) of those coefficients of terms of the samedegree.

More specifically, perform addition—i.e., parity check—of thecoefficient R^(i) _(m) of a remainder r^(i)(x) of specific degree withthe data bit a_(i) being at “1” in GF(2) per each m; then, let theresult be a check bit b_(m).

An explanation will next be given of a calculation method duringdecoding as required when performing the selection of “0”-fixed bitpositions of 111 bits.

Firstly, the calculation of syndrome polynomial S₁(x) is done, which isfor obtaining a division remainder at m₁(x) of polynomial ν(x) with datad_(i) as read out of a memory cell being as its coefficient. Thisarithmetic operation is performed, as shown in FIG. 9, by multiplicationof d_(i) and the coefficient P^(i) _(m) (m is 0, 1, . . . , 7) of theremainder p^(i)(x) that is obtained by dividing x_(i) (i is 254, 253, .. . , 0) by m₁(x), and addition (parity check) of its result. In otherwords, let a parity check result of m-th degree coefficient P^(i) _(m)of p^(i)(x) with d_(i)=“1” be the m-degree coefficient of the syndromepolynomial S₁(x).

In this calculation procedure, no calculations are done for portions ofd_(i)=“0” and when P^(i) _(m) is “0”, so that the selection ofout-of-use bits determines the calculation amount in the case where allof 239 information bits are not used.

A calculation method relating to another syndrome polynomial S₃(x) willnext be described below.

In relation to the syndrome polynomial S₃(x), what is needed forsearching the error location j is the syndrome polynomial S₃(x³). Notethat S₃(x) per se is a division remainder at m₃(x) of ν(x) Between ν(x³)and S₃(x³), a relationship shown in the following Expression 17 isestablishable, where P^(i) _(m) (m=0, 1, . . . , 7) is the coefficientof remainder polynomial p^(i)(x) obtained by dividing x^(i) by m₁(x),and d_(i) is the coefficient of x^(i).

S ₃(x)≡ν(x)mod m ₃(x),

m ₃(x ³)≡0 mod m ₁(x),

S ₃(x ³)≡ν(x ³)mod m ₃(x ³)≡ν(x ³)mod m ₁(x),

ν(x)≡Σd _(i) x ^(i),

ν(x ³)≡Σd _(i) x ^(3i),

x ^(i) ≡p ^(i)(x)mod m ₁(x),

x ^(3i) ≡p ^(3i)(x)mod m ₁(x),

S ₃(x ³)≡ν(x ³)mod m ₁(x)≡Σd _(i) p ^(3i)(x)mod m ₁(x).  [Exp. 17]

From the above-described Expression 17, S₃(x³) is calculable by thecoefficient d_(i) of ν(x) and the remainder p^(3i)(x). Thus, what isneeded here is the coefficient P^(i) _(m) of p^(i)(x) at m₁(x) of x^(i),and its practical calculation method is as shown in FIG. 10.

In this calculation process also, no calculations are done for d_(i)=“0”portions and when P^(3i) _(m) is “0”, so the selection of out-of-usebits determines the calculation quantity in case all of the 239information bits are not used. As the decoding includes a process ofperforming computation for searching the error position(s) aftercompletion of the syndrome polynomial calculation, the calculationamount is desirably minimized in order to shorten a time taken for suchcalculation. This is attainable by performing selection of 128 optimalterms (degrees) from the 238-degree information polynomial f(x). Thisselection method will next be described below.

For the syndrome polynomials S₁(x) and S₃(x³), calculations areperformed simultaneously in a parallel way. Calculation of eachdegree-term in each polynomial is the parity check of “1”; thus, thetotal calculation amount is expected to decrease if the coefficient ofevery degree is calculated without appreciable variations within almostthe same length of time.

One preferred selection method therefore is arranged to include thesteps of obtaining, for each “n”, a total sum of those with thecoefficients being at “1” of these syndrome calculation-used 7-degreeremainder polynomials p^(n)(x) and p^(3n)(x), and then selecting aspecific number of n's corresponding to the required data bit numberfrom the least side in number of the total sum. In this event, the firstsixteen ones, i.e., the coefficients of x⁰ to x¹⁵, are used as the checkbits to be fixed, and perform ascending-order selection of a total sumof “1”s of the coefficients to select 128 terms from the seventeenth oneet seq.

Additionally, upon completion of the selection within a group of thesame total-sum numbers, selection is done in order from the overlap of“1”s being less at the same degree terms as the reference whilespecifying n's as a reference with the coefficients “1” being uniformlydistributed between respective degree terms within p^(n)(x) andp^(3n)(x) and then letting these n's be the reference. In other words,selection is done in order from the least side of the total sum ofcoefficients in the same terms as that of the reference withcoefficients “1” of p^(n)(x), p^(3n) (x).

FIG. 11 shows 144 degrees “n” for use in the case of 144-bit dataselected from 254 degrees in data polynomial f(x)x¹⁶ as described above.

Although this selection method does not always minimize the greatest oneof the number of the coefficients “1” of respective degrees of thepolynomial for execution of parity checking, it is still a simple methodcapable of reducing a step number of syndrome calculation while at thesame time reducing the scale of syndrome calculator circuitry withoutrequiring large-scale calculation including search-up of a calculationstep-minimized one from among all possible combinations.

FIG. 12 shows a coefficient table of remainder polynomial r^(n)(x)obtained by g(x), i.e., a table of degree number “n”, at which thecoefficient of remainder r^(n)(x) for selected x^(n) is “1”.

For example, the degree number “n” of r^(n)(x) with the coefficient ofx¹⁵ being “1” is 17, 18, 22, . . . , 245, 249 and 250 written in fieldsdefined by the number of coefficient “1” being from 1 to 62, in thecolumn of m=15. Note that b₁₅ which is equivalent to the coefficient ofa check bit x¹⁵ is obtainable as a result of parity check of thisselected n-degree terms' coefficients in the information data polynomialf(x) x¹⁶.

FIG. 13 shows an exemplary circuit, which performs check bit calculationbased on the table shown in FIG. 12. It is apparent from the table ofFIG. 12 that for m=11, 5, 2 of x^(m), there are a maximum of 72 bitnumbers to be subjected to parity check. So, this case is shown as anexample in FIG. 13. Since there are indicated in the table such thedegree numbers “n” that the coefficient R^(n) _(m) of m-degree term ofremainder polynomial r^(n)(x) is not “0”, select “n” from the table foreach “m”, and then execute parity check using a_(n) or /a_(n).

An appropriate combination of parity check (PC) circuits to be used isdetermined depending on the number of inputs belonging to which one ofthe division remainder systems of four (4). More specifically, if it isdividable by 4, 4-bit PC is solely used. If such division results inpresence of a remainder of 1, 5-bit PC is added. If the remainder is 2,2-bit PC is added. If 3 remains then 3-bit PC is added.

In the example of m=11, 5 and 2, there are 72 inputs. A check bitcalculator circuit adaptable for use in this case is configurable fromthree stages of parity check (PC) circuits as shown in FIG. 13. Aprimary stage consists of eighteen 4-bit PCs. The second stage is of 18inputs, so let it be arranged by four 4-bit PCs and one 2-bit PC. Thethird stage becomes 5 inputs, so this is made up of a one 5-bit PC. Anoutput of the third-stage parity check circuit becomes a check bitb_(m), /b_(m).

Similar calculation to that in the case of check bit calculation will beperformed in the syndrome calculation, in a way set forth below.

FIG. 14 is a table of the number of degrees whose coefficient is “1” in7-degree remainder polynomial p^(n)(x) for use in the calculation of thesyndrome polynomial S₁(x). For example, the degree number n of p^(n)(x)with the coefficient of x⁷ being “1” is 7, 11, 12, . . . , 237, 242, 245written in fields defined by the number of coefficient “1” being from 1to 56, in the column of m=7. The coefficient of x⁷ of S₁(x) is obtainedas a result of parity check of the coefficient of this selected n-degreeterm in the data polynomial ν(x).

An example of electrical circuitry for performing calculation of thesyndrome S₁(x) based on the table of FIG. 14 is shown in FIG. 15. It isapparent from FIG. 14 that in the case of m=6, 2, the number of bits tobe parity-checked is 66 in maximum. This case is shown as an example inFIG. 15. As those degree numbers n with the coefficient P^(n) _(m) ofm-degree term of remainder polynomial p^(n)(x) not being at “0” arelisted in the table, select n from this table about each m and thenexecute parity check using data bits d_(n) and /d_(n).

A proper combination of parity checkers (PCs) used is determineddepending on the number of inputs belonging to which one of the divisionremainder systems of 4. More precisely, if it is just dividable by 4,then 4-bit PC is solely used; if the division results in presence of aremainder 1, 5-bit PC is added; if the remainder is 2, 2-bit PC isadded; and, if 3 remains then 3-bit PC is added.

In the example of m=6, 2, there are 66 inputs. So in this case also,three stages of parity check (PC) circuits are used to configure theintended calculator circuitry. The first stage is made up of sixteen4-bit PCs and one 2-bit PC. The second stage is of 17 inputs, so it isconstituted from three 4-bit PCs and a 5-bit PC. The third stage is of 4inputs and thus is arranged by only one 4-bit PC. An output of the thirdstage becomes a coefficient (s1)m.

The same goes with the calculation of the syndrome polynomial S₃(x³),and this will be discussed below.

FIG. 16 is a table of the number of degrees whose coefficient is “1” inthe remainder p^(3n)(x) for use in the calculation of syndromepolynomial S₃(x³).

For example, the degree number n of p^(3n)(x) with the coefficient of x⁷being “1” is 4, 8, 14, . . . , 241, 242 and 249 written in fields from 1to 58 in the column of m=7. A data (s3)₇ that corresponds to thecoefficient of x⁷ of S₃(x³) is obtainable as a result of parity check ofthe coefficient of this selected n-degree term in the data polynomialν(x).

An exemplary calculation circuit therefor is shown in FIG. 17. As theFIG. 16 table suggests that the number of bits to be parity-checked form=5 of x^(m) is 73 in maximum, this case is shown in FIG. 17 as anexample. Since those degree numbers n with the coefficient P^(3n) _(m)of m-degree term of remainder polynomial p^(3n)(x) not being at “0” areshown in the table, select n from the table for each m and then executeparity check using d_(n) and /d_(n).

An appropriate combination of parity checkers (PCs) used is determinabledepending on the number of inputs belonging to which one of the4-division remainder systems. More specifically, if dividable by 4, use4-bit PC only. If the division results in presence of a remainder 1, add5-bit PC. If the remainder is 2, add 2-bit PC. If 3 remains then add3-bit PC.

In the example of m=5, there are 73 inputs. Therefore, in this casealso, three stages of parity check circuits are used to configure thecalculation circuitry. The first stage is made up of seventeen 4-bit PCsand a 5-bit PC. As the second stage is of eighteen inputs, it is formedof four 4-bit PCs and a 2-bit PC. The third stage may be configured fromonly one 5-bit PC as it becomes five inputs. An output of the thirdstage becomes the coefficient (s3)_(m), /(s3)_(m).

Next, it will be explained a method of making an error locationsearching circuit small in scale, which performs index comparison tosearch an error location(s).

A required calculation is to solve the index congruence shown inExpression 14. In detail, it is in need of solving two congruences.Firstly, obtain y_(n) of y²+y+1=α^(y n) based on the syndrome index.Then, find the index n of y=α^(n) corresponding to y_(n) based on thecorresponding relationship, and solve the index i of x based onx=α^(σ1)y.

The congruences each is formed on GF(256), i.e., mod 255. If directlyexecuting this calculation as it is, it becomes equivalent to performingthe comparison of 255×255, and resulting in that the circuit scalebecomes large. In this embodiment, to make the circuit scale small, thecalculation will be performed in parallel.

That is, 2^(n)−1=255 is factorized into two prime factors M and N (i.e.,first and second integers, respectively), and each congruence is dividedinto two congruences. Then, it will be used such a rule that in case anumber satisfies simultaneously the divided congruences, it alsosatisfies the original congruence. In this case, to make the circuitscale and calculation time as small as possible when the congruencecalculations are executed in parallel, it is preferred to make thedifference between the two integers as small as possible.

In detail, 255 is dividable as 17×15, 51×5 or 85×3. In this embodiment,17×15 is used, and two congruences will be solved simultaneously withmod 17 and mod 15.

First, to obtain y_(n), congruences shown in Expression 18 are used.That is, addition/subtraction with mod 17 between indexes multiplied by15 and addition/subtraction with mod 15 between indexes multiplied by 17are performed simultaneously in parallel.

$\begin{matrix}{\mspace{20mu} \begin{matrix}{{15y_{n}} \equiv {{15\sigma_{3}} - {45\sigma_{1}}}} & \left( {\text{mod}\mspace{14mu} 17} \right) \\{{17y_{n}} \equiv {{17\sigma_{3}} - {51\sigma_{1}}}} & \left( {\text{mod}\mspace{14mu} 15} \right) \\{->{y_{n} \equiv {\sigma_{3} - {3\sigma_{1}}}}} & \left( {\text{mod}\mspace{14mu} {15 \cdot 17}} \right)\end{matrix}} & \text{[Exp.~~18]}\end{matrix}$

To obtain index i, the following Expression 19 is used. Here also,addition/subtraction with mod 17 between indexes multiplied by 15 andaddition/subtraction with mod 15 between indexes multiplied by 17 areperformed simultaneously in parallel.

$\begin{matrix}\begin{matrix}{{15i} \equiv {{15n} + {15\sigma_{1}}}} & \left( {\text{mod}\mspace{14mu} 17} \right) \\{{17i} \equiv {{17n} + {17\sigma_{1}}}} & \left( {\text{mod}\mspace{14mu} 15} \right) \\{->{i \equiv {n + \sigma_{1}}}} & \left( {\text{mod}\mspace{14mu} {17 \cdot 15}} \right)\end{matrix} & \left\lbrack {{Exp}.\mspace{14mu} 19} \right\rbrack\end{matrix}$

To execute addition and subtraction operations described above, anoperation circuit is used, which is referred to as an index rotatordescribed later. This circuit has a scale of product of numbers countingthe different indexes with respect to mod, which are subjected toaddition or subtraction. Therefore, if the above-described division ofthe congruence is not done, the calculation scale of the firstcongruence becomes 255×85=21675; and that of the second congruence255×255=65025.

By contrast, by use of the above-described congruence division, thecircuit scale of the first congruence becomes 17×17=289 plus 15×5=75,i.e., 364, which is a scale of about 1.7%. The circuit scale of thesecond congruence becomes 17×17=289 plus 15×15=225, i.e., 514, which isa scale of about 0.8%. As described above, the calculation scale is madesmall; and the calculation time shortened.

Next, the relationships between the respective indexes will besummarized below.

FIG. 18 shows the coefficient “1”, “0” at every degree number n ofpolynomial p^(n)(x), which is the reminder of division of x^(n) bym₁(x), and hexadecimal indications thereof of degrees 0 to 3 and 4 to 7.All index relationship explained below will be calculated based on thistable. That is, each of four basic operations for the root a of m₁(x),which are performed by use of indexes, always corresponds to anywhere inthis table with the relationship of one versus one.

FIGS. 19A and 19B show, with respect to the index range of n=0 to 255,index y_(n) of y²+y+1=α^(yn), coefficients of the correspondingpolynomial and hexadecimal indications thereof. In case two “n”scorrespond to the same y_(n), the number of errors is 2 while in caseone “n” corresponds to one y_(n), there is one error. In case the numberof errors is 2, it is shown that two indexes constituting a pair.

Note here that y²+y+1=0 at indexes 85 and 170. This means that S₃/S₁ ³is 0 element of GF(256). In this case, there will be provided a controlsystem, which directly controls an error correction circuit by use ofthe syndrome value.

FIG. 20 shows summarized relationships between index n and y_(n), inwhich two tables are arranged in parallel as follows: in one table,y_(n) is arranged in order of n; in the other, n is arranged in order ofy_(n). In the latter table, it is shown that two “n”s correspond to thesame y_(n) except y_(n)=0. At n=85 and n=170, there is no correspondingy_(n) (i.e., corresponding to element 0 of Galois field).

FIG. 21 shows that each index y_(n) is uniquely classified by15y_(n)(mod 17) and 17y_(n)(mod 15). In the left half of FIG. 21, y_(n)is arranged from the least side of 15y_(n)(mod 17) while in the righthalf, y_(n) is arranged from the least side of 17y_(n)(mod 15).

It will be apparent from the table shown in FIG. 21 that congruence'sparalleling as shown in Expression 18 leads to reduction of thecalculation scale. The calculations for the congruences of 15y_(n)(mod17) and 17y_(n)(mod 15) bring a common element, that becomes y_(n)obtained from the syndrome index.

FIG. 22 shows the relationship between indexes i selected as data bitsand the corresponding physical bit locations k, and the indexclassification by 15i(mod 17) and 17i(mod 15). It is apparent from FIG.22 that index i is classified into a pair defined by 15i(mod 17) and17i(mod 15). It will be apparent from the table shown in FIG. 22 thatcongruence's paralleling as shown in Expression 19 leads to reduction ofthe calculation scale.

The calculations for the congruences of 15i(mod 17) and 17i(mod 15)bring a common element, that becomes i (i.e., k) obtained from index nand that of syndrome S₁ when the physical bit location is calculated.

FIG. 23A shows an index rotator, which cyclically exchanges the passagesfrom input nodes to output nodes to operate addition and subtraction ofindexes; and FIG. 23B circuit symbol thereof. Since “subtraction”between indexes corresponds to the reversed shift in case of “addition”,addition will be explained below. Addition of indexes corresponds toproduct in the expression of root α. Therefore, two inputs of therotator are expressed as α^(σi) and α^(σj) and output as α^(σi+σj).

As shown in FIG. 23A, the index rotator is for transferring inputs α⁰,α₁, α², . . . , α²⁵⁴ on the input nodes 201 to any of the output nodes202 by use of only switch circuits 203 and wirings. Here is shown such acase that there are input and output nodes corresponding to all elementon GF(256). In this case, input α⁰ may be transferred to any one of theoutput nodes 202 correspond to α⁰, α₁, α², . . . α²⁵⁴ via the transistorswitch circuits 203.

The gates of the switch circuit transistors serve as control input nodes204, to which multipliers and divisors are input. That is, invertedsignals of α⁰, α₁, α², . . . , α²⁵⁴ being used as control signals, atransistor corresponding to indexes to be added will be tuned on. Asshown in FIG. 23A, in case N channel transistors are used as switchingtransistors, the input/output nodes are precharged, and the controlsignals controls which discharge passage becomes active between theinput nodes and the output nodes.

Although, in the explanation described above, all element on GF(256) isused, element numbers relating to addition and subtraction aresignificantly different as dependent on what addition or subtraction isperformed between indexes. In consideration of this, the solution methodof a congruence is divided into two parts in parallel, thereby makingthe circuit scale small.

Next, it will be explained examples adapted practically to an errorlocation searching part.

Index rotator 200 a shown in FIG. 24 is for operating the right side ofthe first congruence (i.e., 15y_(n)≡15σ₃−45σ₁(mod 17)) in Expression 18.Input and control input are σ₃ and σ₁, respectively. Disposed on theinput node side are 17 drivers 211 a, which decode coefficients (s3)m(m=0 to 7) of 7-degree polynomial obtained by the syndrome calculationto drive an input signal at an index position of 15σ₃(17), i.e., theremainder class of 15σ₃ obtained by modulo 17.

Disposed on the control input side are 17 drivers 212 a, which decodecoefficients (s1)_(m) (m=0 to 7) of 7-degree polynomial obtained by thesyndrome calculation to drive a control input signal at an indexposition of −45σ₁(17), i.e., the remainder class of −45σ₁ obtained bymodulo 17.

With this index rotator 200 a, an index corresponding to 15y_(n)(17) inthe table described above, i.e., the remainder class of 15y_(n) obtainedby modulo 17, is output to the output nodes.

FIG. 25 shows decoder circuits used in the drivers 211 a and 212 a shownin FIG. 24, each of which is formed of NAND circuits arranged inparallel in number of the irreducible remainder polynomials p^(n)(x)belonging to each remainder class. Each NAND circuit is formed oftransistors connected in series with gates thereof being selectivelyapplied with syndrome coefficients of m=0 to 7.

Precharge transistors are driven by clock CLK to precharge thecorresponding common nodes. Whether the common nodes are discharged ornot will serve as index signals of the remainder classes. Coefficientsignal wirings and the inverted signal ones are disposed so as toconstitute pairs, and these are selectively coupled to the gates oftransistors in the NAND circuits in accordance with decoding codes.

The number of NAND nodes coupled in parallel is: 15 in case modulus ofthe remainder class is 17; and 17 in case that is 15.

Similar decode circuit is disposed on the control signal input nodeside, decoder code of which is shown in FIG. 26. In this table, indexesn of the irreducible remainder polynomial p^(n)(x) are classified intothe remainder classes 15n(17), which are obtained by modulo 17 with nbeing multiplied by 15. The remainders are classified by indexes 0 to16, each of which includes 15 n's. In accordance with these coefficientsof indexes of the corresponding p^(n)(x), it will be determined whichsignal wirings are coupled to decode transistor gates.

For example, in case of index 1, NAND nodes to be coupled in parallelare those of: n=161, 59, 246, 127, 42, 93, 178, 144, 212, 229, 110, 195,8, 76 and 25.

FIG. 27 shows such a table that indexes n of the irreducible remainderpolynomial p^(n)(x) are classified into the remainder classes−45^(n)(17), which are obtained by modulo 17 with n being multiplied by−45. The remainders are classified by indexes 0 to 16, each of whichincludes 15 n's. In accordance with these coefficients of indexes of thecorresponding p^(n)(x), it will be determined which signal wirings arecoupled to decode transistor gates.

For example, in case of index 1, NAND nodes to be coupled in parallelare those of: n=88, 173, 122, 156, 71, 20, 190, 207, 241, 54, 37, 139,105, 224 and 3.

Index rotator 200 b shown in FIG. 24 is for operating the right side ofthe second congruence (i.e., 17y_(n)≡17σ₃−5σ₁ (mod 15)) in Expression18. That is, this is for obtaining 17σ₃−51σ₁ (mod 15) from indexes σ₃and σ₁.

Inputs are σ₃. To decode coefficients (s3)_(m) (m=0 to 7) of 7-degreepolynomial obtained by the syndrome calculation, and drive an inputsignal at an index position of 15σ₃(17), 15 drivers 211 b are disposed.

Control inputs are σ₁. To decode coefficients (s1)_(m) (m=0 to 7) of7-degree polynomial obtained by the syndrome calculation, and drive acontrol input signal at an index position of −51σ₁(15), 5 drivers 212 bare disposed. Since modulus is 15, 51 and 15 have a common prime 3.Therefore, remainder class number is 5 that is obtained by dividing 15by 3, and the indexes of the remainder by modulo 15 are 0, 3, 6, 9, and12. As a result, the number of drivers 212 b is 5; and that of controlinputs is also 5.

Output to the output nodes are indexes, which designate the classescorresponding 17y_(n)(15) in the above-shown table.

Decode circuits in the drivers are the same as shown in FIG. 24 exceptthat input code is different from it.

FIG. 29 shows such a table that indexes n of the irreducible remainderpolynomial p^(n)(x) are classified into the remainder classes 17n(15),which are obtained by modulo 15 with n being multiplied by 17. Theremainders are classified by indexes 0 to 14, each of which includes 17n's. In accordance with these coefficients of indexes of thecorresponding p^(n)(x), it will be determined which signal wirings arecoupled to decode transistor gates.

For example, in case of index 1, NAND nodes to be coupled in parallelare those of: n=173, 233, 203, 23, 83, 158, 188, 68, 38, 128, 143, 98,53, 218, 8, 113 and 248.

FIG. 30 shows such a table that indexes n of the irreducible remainderpolynomial p^(n)(x) are classified into the remainder classes −51n(15),which are obtained by modulo 15 with n being multiplied by −51. Theremainders are classified by indexes 0, 3, 6, 9 and 12, each of whichincludes 51 n's. In accordance with these coefficients of indexes of thecorresponding p^(n)(x), it will be determined which signal wirings arecoupled to decode transistor gates.

For example, in case of index 3, NAND nodes to be coupled in parallelare those of: n=232, 22, 117, 122, 62, . . . , 47, 52, 27 and 2.

FIG. 31 shows bus structure, which integrates the operation results ofthe above-described two index rotators 200 a and 200 b to outputoperation signals to the following stage. To output the output signalsof the rotators 200 a and 200 b, there are prepared output buses 215 aand 215 b, which are formed of 17 and 15 wirings, respectively.

With these buses 215 a and 215 b, 17+15 index data are transferred tothe following decode part, which detects an error location(s) defined bythe index of y. Here is shown that the operation result is inverted, anda selected index is output as a “H” signal.

Based on the signals on the output buses 215 a and 215 b, such decodingis performed as: from {15y_(n)(17), 17y_(n)(15)} to y_(n); from y_(n) ton; and from n to 15n(17), 17n(15), and the next state calculation isfollowed.

FIG. 32 shows index rotator 300 a, which operates the right side of thefirst congruence (i.e., 15i≡15n+15σ₁(mod 17)) in Expression 19. Inputsare signals on buses 215 a and 215 b. To decode these signals and inputindexes of 15n by modulo 17, 17 drivers 311 a are disposed.

Control inputs are σ₁. Disposed on the control input nodes 17 are 17drivers 311 b, which decode coefficients (s1)_(m) (m=0 to 7) of 7-degreepolynomial obtained by the syndrome calculation to drive a control inputsignal at an index position of 15σ₁(17), i.e., the remainder class of15σ₁ obtained by modulo 17.

Obtained at the output nodes are indexes designating the remainder classof 15i by modulo 17 corresponding to 15i(17).

FIG. 33 shows decode circuits in the drivers 311 a disposed on the inputside in FIG. 32. there are disposed a certain number of NAND circuitscoupled in parallel with transistors connected in series, the numberbeing determined in accordance with the relationships between theremainder classes. The gates of the NAND circuits are selectivelyapplied with the signals corresponding to the remainder class indexes17y_(n)(15) and 15y_(n)(17), which are output from the above-describedrotators 200 a and 200 b. Each common node of NAND circuits connected inparallel, is precharged by precharge transistor driven by control signal/pr. Whether the common node is discharged or not serves as the indexsignal of a selected remainder class.

Although, in FIG. 33, some transistors in NAND circuit are shown as oneswithout gates, there are in practice no transistors at these positions,and only wirings are disposed. That is, practically connected in seriesin each NAND circuit are only two transistors.

Next, the relationships between remainder classes and codes serving asdecoder inputs will be explained below.

FIG. 34 shows the relationships between the remainder class indexes15y_(n)(17), 17y_(n)(15) and 15n(17), and classes of y_(n) and ncorresponding thereto.

For example, {15y_(n)(17), 17y_(n)(15)} corresponding to the remainderclass 15n(17)=1 is as follows: {0, 7}, {1, 14}, {2, 13}, {4, 3}, {4, 4},{7, 0}, {7, 11}, {8, 1}, {8, 14}, {11, 3}, {11, 13}, {12, 6}, {12, 12},{15, 11} and {15, 14}. OR connections of these become decoder outputs.

That is, decoding is performed in accordance with this table, andoutputs thereof become inputs of the index rotator 300 a.

FIG. 35 shows index rotator 300 b, which operates the right side of thesecond congruence (i.e., 17i≡17n+17σ₁(mod 15)) in Expression 19. Inputsare signals on buses 215 a and 215 b. To decode these signals and inputindexes of 17n in modulo 15, 15 drivers 311 b are disposed.

Control inputs are σ₁. Disposed on the control input nodes are 15drivers 312 b, which decode coefficients (s1)_(m) (m=0 to 7) of 7-degreepolynomial obtained by the syndrome calculation to drive a control inputsignal at an index position of 17σ₁(15), i.e., the remainder class of17σ₁ obtained by modulo 15.

Obtained at the output nodes are indexes designating the remainder classof 17i by modulo 15 corresponding to 17i(15).

Decoder circuits used in the drivers are the same as shown in FIG. 33,and codes thereof are shown in FIG. 36. That is, FIG. 36 is a tableshowing the relationships between the remainder class indexes15y_(n)(17), 17y_(n)(15) and 17n(15), and classes of “y_(n)” and “n”corresponding thereto.

For example, {15y_(n)(17), 17y_(n)(15)} corresponding to the remainderclass 17n(15)=1 is as follows: {0, 10}, {1, 1}, {1, 13}, {2, 9}, {3, 1},{4, 3}, {5, 12}, {6, 3}, {6, 8}, {11, 3}, {11, 8}, {12, 12}, {13, 3},{14, 1}, {15, 9}, {16, 1} and {16, 13}. OR connections of these becomedecoder outputs.

That is, decoding is performed in accordance with this table, andoutputs thereof become inputs of the index rotator 300 b.

FIG. 37 shows such a part that integrates the operation results of theindex rotators 300 a and 300 b, and detecting error location y as a bitposition. Outputs of the index rotators 300 a and 300 b are output to15i(17) bus 315 a and 17i(15) bus 315 b, respectively.

It is apparent from the table shown in FIG. 22, which shows therelationship between “k”, “i”, 15i(17) and 17i(15), that “i” is uniquelydesignated by a pair of signals on these buses 315 a and 351 b. It ispossible to specify “k” from the combination of {15i(17), 17i(15)}.Therefore, “k” will be selected via decoder circuits 316 each havingtwo-input NOR gate. α^(i) becomes the final output of the operationresult. One or two “k” is selected, and it designates error locations upto 2 bits.

FIG. 38 shows a summarized relationship between 15i(17), 17i(15), “i”and “k”, in which bit position indexes “i” are arranged in order ofphysical bit positions “k”, and remainder class indexes 15i(17) and17i(15) also are shown as corresponding to the respective “i”.

A circuit for finally error-correcting based on the above-describedoperation result is shown in FIG. 39. The error correction circuit hasdifferent operations from each other in accordance with the syndromeoperation results. In case S₁×S₃ is not 0, one or two errors have beengenerated, and error correction is performed. In case of S₁×S₃=0, thereare two modes as follows: if s_(l)=s₃=0, there are no errors, and it isno need of error-correcting; and if one of S₁ and S₃ is 0, there arethree or more errors, and it is not correctable.

To judge these situations, a judge circuit 401 with NOR gates G1 and G2is prepared for detecting such a situation that all of the coefficients(s1)_(m) and (s3)_(m) of the syndrome polynomials is “0”. In case thereare three or more error bits, one of the outputs of NOR gate G1 and G2becomes “0”, and in response to it, NOR gate G5 outputs “1” thatdesignates a non correctable state. In this case, NOR gate G4 outputs“0”, and this makes the decode circuit 402 performing error-correctioninactive.

In case there are no errors, both outputs of NOR gates G1 and G2 are“1”, and NOR gates G4 and G5 also output “0”, thereby making the decodecircuit 402 inactive.

If there is one bit error or two bit errors, both gates G1 and G2 output“0”, and output “1” of NOR gate G4 makes the decode circuit 402 with NORgates G6 and G7 active. To invert dada d_(k) at the bit position kselected by α^(i), an inverting circuit 403 is prepared, which uses atwo-bit parity check circuit shown in FIG. 40. With this invertingcircuit 403, in case there are no errors, data d_(k) is output as it iswhile data d_(k) is inverted to be output at the error position.

According to this embodiment, the calculation scale and time of the ECCcircuit with 2EC-BCH code will be effectively made small. That is, toperform error location searching in this embodiment, product andquotient operations of elements of GF(256) are executed as addition andsubtraction operations with respect to indexes of the elements. In thiscase, 255 is divided into prime factors 15 and 17, and object numbersmultiplied by 15 are subjected to addition/subtraction by modulo 17;other object numbers multiplied by 17 are subjected toaddition/subtraction by modulo 15. These addition/subtraction operationsare performed in parallel, and the final addition/subtraction result bymod 255 will be obtained from the results of two systems of theaddition/subtraction operations.

The circuit scale of the index rotators for performingaddition/subtraction of indexes will be reduced to about 2% or less incomparison with the case where the addition/subtraction circuit isformed as it is as performed by mod 255.

FIG. 41 shows a functional block configuration of ECC system 8 in theembodiment described above, in which memory core 10 is shown as oneblock. In encoding part 81, data polynomial f(x)x¹⁶ based on theinformation polynomial f(x) is divided by code generation polynomialg(x)=m₁(x)m₃(x) to generate check bits, which are written into thememory core 10 together with to-be-written data.

Read out data from the memory core 10, which is expressed by ν(x), areinput to syndrome operation part 82 to be subjected to syndromeoperations. In the error location searching part 83 for searching errorlocation(s) based on the obtained syndrome coefficients, paralleloperations are performed with index rotators 200 a and 200 b; andparallel operations for the operation results are further performed withindex rotators 300 a and 300 b.

That is, index rotators 200 a and 200 b are for calculating thecongruences shown in Expression 18, which compare index σ₃−3σ₁ withindex y_(n) obtained based on the variable conversion of: x=α^(σ1) tofind index y_(n) corresponding to an error location. Index rotators 300a and 300 b are for calculating the congruences shown in Expression 19,which restore index y_(n) to a real index “i” corresponding to the errorlocation.

Error correcting part 84 inverts bit data at the error position.

This invention is applicable to any types of electrically erasableprogrammable semiconductor memory devices other than the flash memory asdiscussed in the above-noted embodiment.

FIG. 42 shows a configuration of one typical memory core circuit in astandard NAND flash memory, to which this invention is adaptable. Thismemory core circuit includes cell array 1 a, sense amplifier circuit 2 aand row decoder 3 a. Cell array 1 a is configured from parallelcombination of NAND cell units (NAND strings) each having a serialconnection of memory cells M0 to M31. NAND cell unit NU has one endconnected to an associated bit line BLe (BLo) through a select gatetransistor S1 and the other end coupled a common source line CELSRC viaanother select gate transistor S2.

The memory cells have control gates, which are connected to word linesWL0-WL31. The select gate transistors S1-S2 have their gates coupled toselect gate lines SGD and SGS, respectively. Word lines WL0-WL31 andselect-gate lines SGD-SGS are driven by the row decoder 3 a.

The sense amp circuit 2 a has one page of sense units SA for performing“all-at-a-time” writing and reading. Each sense unit SA is associatedwith a bit line selector circuit 4, which selects either one of adjacentbit lines BLe, BLo for connection thereto. With such an arrangement,memory cells simultaneously selected by a single word line WLi and aplurality of even-numbered bit lines BLe (or odd-numbered bitlines BLo)constitute a page (one sector), which is subjected to a collectivewriting/reading. Bit lines on the non-select side are used as shieldlines with a prespecified potential being given thereto. This makespossible to suppress unwanted interference between the presentlyselected bit lines.

A group of NAND cell units sharing the word lines WL0-WL31 makes up ablock, i.e., a unit for data erasure. As shown in FIG. 42, apredetermined number, n, of blocks BLK0-BLKn are laid out in theextending direction of the bit lines.

In the NAND flash memory with the core circuit also, the need grows foron-chip realization of correction of 2 bits or more errors with advancesin the miniaturization and in per-cell multi-level storage scheme. Theerror correction system incorporating the principles of this inventionis capable of reducing or minimizing the calculation scale for errordetection and correction, thereby enabling successful achievement ofcomputation at high speeds. Thus it can be said that the ECCarchitecture unique to the invention offers advantages upon applicationto memory chips of the type stated supra.

Since the above-described parallel operation method uses a generalproperty with respect to the remainder class of the finite Galois field,it is not limited to 2EC-BCH system on GF(256), but is able to beadapted to other systems. For example, t(≧2)-error correcting BCH codemay be used in general. In this case, it becomes such a BCH code onGF(2^(n)) that has t-element of α, α³, . . . , α^(2t−1) as rootsthereof.

[Application Devices]

As an embodiment, an electric card using the non-volatile semiconductormemory devices according to the above-described embodiments of thepresent invention and an electric device using the card will bedescribed bellow.

FIG. 43 shows an electric card according to this embodiment and anarrangement of an electric device using this card. This electric deviceis a digital still camera 101 as an example of portable electricdevices. The electric card is a memory card 61 used as a recordingmedium of the digital still camera 101. The memory card 61 incorporatesan IC package PK1 in which the non-volatile semiconductor memory deviceor the memory system according to the above-described embodiments isintegrated or encapsulated.

The case of the digital still camera 101 accommodates a card slot 102and a circuit board (not shown) connected to this card slot 102. Thememory card 61 is detachably inserted in the card slot 102 of thedigital still camera 101. When inserted in the slot 102, the memory card61 is electrically connected to electric circuits of the circuit board.

If this electric card is a non-contact type IC card, it is electricallyconnected to the electric circuits on the circuit board by radio signalswhen inserted in or approached to the card slot 102.

FIG. 44 shows a basic arrangement of the digital still camera. Lightfrom an object is converged by a lens 103 and input to an image pickupdevice 104. The image pickup device 104 is, for example, a CMOS sensorand photoelectrically converts the input light to output, for example,an analog signal. This analog signal is amplified by an analog amplifier(AMP), and converted into a digital signal by an A/D converter (A/D).The converted signal is input to a camera signal processing circuit 105where the signal is subjected to automatic exposure control (AE),automatic white balance (AWB) control, color separation, and the like,and converted into a luminance signal and color difference signals.

To monitor the image, the output signal from the camera processingcircuit 105 is input to a video signal processing circuit 106 andconverted into a video signal. The system of the video signal is, e.g.,NTSC (National Television System Committee). The video signal is inputto a display 108 attached to the digital still camera 101 via a displaysignal processing circuit 107. The display 108 is, e.g., a liquidcrystal monitor.

The video signal is supplied to a video output terminal 110 via a videodriver 109. An image picked up by the digital still camera 101 can beoutput to an image apparatus such as a television set via the videooutput terminal 110. This allows the pickup image to be displayed on animage apparatus other than the display 108. A microcomputer 111 controlsthe image pickup device 104, analog amplifier (AMP), A/D converter(A/D), and camera signal processing circuit 105.

To capture an image, an operator presses an operation button such as ashutter button 112. In response to this, the microcomputer 111 controlsa memory controller 113 to write the output signal from the camerasignal processing circuit 105 into a video memory 114 as a flame image.The flame image written in the video memory 114 is compressed on thebasis of a predetermined compression format by a compressing/stretchingcircuit 115. The compressed image is recorded, via a card interface 116,on the memory card 61 inserted in the card slot.

To reproduce a recorded image, an image recorded on the memory card 61is read out via the card interface 116, stretched by thecompressing/stretching circuit 115, and written into the video memory114. The written image is input to the video signal processing circuit106 and displayed on the display 108 or another image apparatus in thesame manner as when image is monitored.

In this arrangement, mounted on the circuit board 100 are the card slot102, image pickup device 104, analog amplifier (AMP), A/D converter(A/D), camera signal processing circuit 105, video signal processingcircuit 106, display signal processing circuit 107, video driver 109,microcomputer 111, memory controller 113, video memory 114,compressing/stretching circuit 115, and card interface 116.

The card slot 102 need not be mounted on the circuit board 100, and canalso be connected to the circuit board 100 by a connector cable or thelike.

A power circuit 117 is also mounted on the circuit board 100. The powercircuit 117 receives power from an external power source or battery andgenerates an internal power source voltage used inside the digital stillcamera 101. For example, a DC-DC converter can be used as the powercircuit 117. The internal power source voltage is supplied to therespective circuits described above, and to a strobe 118 and the display108.

As described above, the electric card according to this embodiment canbe used in portable electric devices such as the digital still cameraexplained above. However, the electric card can also be used in variousapparatus such as shown in FIGS. 45A to 45J, as well as in portableelectric devices. That is, the electric card can also be used in a videocamera shown in FIG. 45A, a television set shown in FIG. 45B, an audioapparatus shown in FIG. 45C, a game apparatus shown in FIG. 45D, anelectric musical instrument shown in FIG. 45E, a cell phone shown inFIG. 45F, a personal computer shown in FIG. 45G, a personal digitalassistant (PDA) shown in FIG. 45H, a voice recorder shown in FIG. 45I,and a PC card shown in FIG. 45J.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A semiconductor memory device comprising an error detection andcorrection system with an error correcting code over Galois fieldGF(2^(n)), wherein the error detection and correction system includes anoperation circuit configured to execute addition/subtraction with modulo2^(n)−1, and wherein the operation circuit includes a first operationpart for performing addition/subtraction with modulo M and a secondoperation part for performing addition/subtraction with modulo N (where,M and N are integers, which are prime with each other as being obtainedby factorizing 2^(n)−1), which perform addition/subtractionsimultaneously in parallel with each other to output an operation resultof the addition/subtraction with modulo 2^(n)−1.
 2. The semiconductormemory device according to claim 1, wherein the error detection andcorrection system is formed to have a 2-bit error correction BCH code,which is configured in such a manner that a certain number of degreesare selected as information bits to be simultaneously error-correctablein the memory device from the entire degree of an information polynomialwith degree numbers corresponding to error-correctable maximum bitnumbers.
 3. The semiconductor memory device according to claim 1,wherein the operation circuit is, for the purpose of obtaining a productor a quotient between elements over Galois field GF(2^(n)), configuredto execute index addition/subtraction of the elements, and wherein thefirst and second operation parts each comprises an index rotator, whichincludes: input nodes, the number of which corresponds to that of oneindexes to be subjected to the index addition/subtraction; output nodesfor outputting the results of the index addition/subtraction; and aswitching circuit having control input nodes, the number of whichcorresponds to that of the other indexes to be subjected to the indexaddition/subtraction, to exchange wiring passages between the inputnodes and the output nodes for selectively connecting the input nodes tothe output nodes.
 4. The semiconductor memory device according to claim1, wherein the error detection and correction system is formed to have a2-bit error correction BCH code over Galois field GF(256), and whereinthe operation circuit is one in an error location searching circuit,which searches index “n” of root α^(n) in GF(256) satisfying a 2-biterror searching equation of: x²+S₁x+(S₃+S₁ ³)/S₁=0 (where, S₁ and S₃ aresyndrome coefficients decoded from read out data of the memory device).5. The semiconductor memory device according to claim 4, wherein theerror location searching circuit comprises index rotators eachincluding: input nodes, the number of which corresponds to that of oneindexes to be subjected to the addition/subtraction; output nodes foroutputting the results of the addition/subtraction; and a switchingcircuit having control input nodes, the number of which corresponds tothat of the other indexes to be subjected to the addition/subtraction,to exchange wiring passages between the input nodes and the output nodesfor selectively connecting the input nodes to the output nodes.
 6. Thesemiconductor memory device according to claim 4, wherein the errorlocation searching circuit comprises first and second operation circuitsserially coupled in such a way that operation results of the former areinput to the latter, and wherein the first operation circuit is forcomparing index σ₃−3σ₁ obtained from syndromes and index y_(n) obtainedbased on the variable conversion of x=α^(σ1)y, thereby searching theindex y_(n) corresponding to an error location, the first operationcircuit comprising: a first index rotator for adding 15σ₃ to −45σ₁ withmodulo 17; and a second index rotator for adding 17σ₃ to −51σ₁ withmodulo 15 simultaneously in parallel with the first index rotator,thereby calculating a congruence of: y_(n)≡σ₃−3σ₁ (mod 255) togetherwith the first index rotator, and wherein the second operation circuitis for restoring the index y_(n) to real index “i” corresponding to theerror location, the second operation circuit comprising: a third indexrotator for adding 15n to 15σ₁ with modulo 17; and a fourth indexrotator for adding 17n to 17σ₁ with modulo 15 simultaneously in parallelwith the third index rotator, thereby calculating another congruence of:i≡n+σ₁ (mod 255) together with the third index rotator.
 7. Thesemiconductor memory device according to claim 1, wherein the memorydevice is a non-volatile semiconductor memory, in which electricallyrewritable and non-volatile memory cells are arranged.
 8. Thesemiconductor memory device according to claim 7, wherein thenon-volatile semiconductor memory includes a cell array with NAND cellunits arranged therein, the NAND cell unit having a plurality of memorycells connected in series.
 9. The semiconductor memory device accordingto claim 7, wherein the non-volatile semiconductor memory stores suchmulti-level data that two or more bits are stored in each memory cell.10. A semiconductor memory device comprising a cell array withelectrically rewritable and non-volatile memory cells arranged therein,and an error detection and correction system, which is correctable up to2-bit errors for read out data of the cell array by use of a BCH codeover Galois field GF(256), wherein the error detection and correctionsystem comprises: an encoding part configured to generate check bits tobe written into the cell array together with to-be-written data; asyndrome operating part configured to execute syndrome operation forread out data of the cell array; an error location searching partconfigured to search error locations in the read out data based on theoperation result of the syndrome operating part; and an error correctingpart configured to invert an error bit in the read out data detected inthe error location searching part, and output it, and wherein the errorlocation searching part includes an operation circuit configured toexecute index addition/subtraction with modulo 255, and wherein theoperation circuit includes a first operation part for performingaddition/subtraction with modulo 17 and a second operation part forperforming addition/subtraction with modulo 15, which performaddition/subtraction simultaneously in parallel with each other tooutput an operation result of the index addition/subtraction with modulo255.
 11. The semiconductor memory device according to claim 10, whereinthe BCH code is configured in such a manner that a certain number ofdegrees are selected as information bits from the entire degree 254 ofan information polynomial with degree numbers corresponding toerror-correctable maximal bit numbers so as to make a calculation scaleof syndrome polynomials as small as possible.
 12. The semiconductormemory device according to claim 10, wherein the first and secondoperation parts each comprises an index rotator, which includes: inputnodes, the number of which corresponds to that of one indexes to besubjected to the addition/subtraction; output nodes for outputting theresults of the addition/subtraction; and a switching circuit havingcontrol input nodes, the number of which corresponds to that of theother indexes to be subjected to the addition/subtraction, to exchangewiring passages between the input nodes and the output nodes forselectively connecting the input nodes to the output nodes.
 13. Thesemiconductor memory device according to claim 10, wherein the errorlocation searching part is for searching index “n” of root α^(n) overGF(256) satisfying an error searching equation of: x²+S₁x+(S₃+S₁ ³)/S₁=0(where, S₁ and S₃ are syndrome coefficients decoded from read out dataof the memory device).
 14. The semiconductor memory device according toclaim 13, wherein the error location searching part comprises indexrotators each including: input nodes, the number of which corresponds tothat of one indexes to be subjected to the addition/subtraction; outputnodes for outputting the results of the addition/subtraction; and aswitching circuit having control input nodes, the number of whichcorresponds to that of the other indexes to be subjected to theaddition/subtraction, to exchange wiring passages between the inputnodes and the output nodes for selectively connecting the input nodes tothe output nodes.
 15. The semiconductor memory device according to claim13, wherein the error location searching part comprises first and secondoperation circuits serially coupled in such a way that operation resultsof the former are input to the latter, and wherein the first operationcircuit is for comparing index σ₃−3σ₁ obtained from syndromes and indexy_(n) obtained based on the variable conversion of x=α_(σ1)y, therebysearching the index y_(n) corresponding to an error location, the firstoperation circuit comprising: a first index rotator for adding 15σ₃ to−45σ₁ with modulo 17; and a second index rotator for adding 17σ₃ to−51σ₁ with modulo 15 simultaneously in parallel with the first indexrotator, thereby calculating a congruence of: y_(n)≡σ₃−3σ₁ (mod 255)together with the first index rotator, and wherein the second operationcircuit is for restoring the index y_(n) to real index “i” correspondingto the error location, the second operation circuit comprising: a thirdindex rotator for adding 15n to 15σ₁ with modulo 17; and a fourth indexrotator for adding 17n to 17σ₁ with modulo 15 simultaneously in parallelwith the third index rotator, thereby calculating another congruence of:i≡n+σ₁ (mod 255) together with the third index rotator.
 16. Thesemiconductor memory device according to claim 10, wherein the cellarray has electrically rewritable and non-volatile memory cells arrangedtherein.
 17. The semiconductor memory device according to claim 16,wherein the cell array has NAND cell units arranged therein, the NANDcell unit having a plurality of memory cells connected in series. 18.The semiconductor memory device according to claim 16, wherein the cellarray stores such multi-level data that two or more bits are stored ineach memory.